This invention relates to measuring probes for use in apparatus in which a determination is made of the position of a surface of a workpiece. Such apparatus includes machine tools and coordinate measuring machines.
A known type of such probe comprises a stylus for contacting the workpiece surface, which is movable in three orthogonal directions x,y,z relative to a probe body. The stylus is biassed to a null position in each of these directions by springs. The probe also contains means for measuring the displacement of the stylus in each of these directions. In use, the probe is mounted in position determining apparatus (such as a machine tool or coordinate measuring machine) on a quill or spindle which is itself movable in the directions x,y,z relative to the workpiece. With the stylus touching the workpiece surface, the coordinates of the point of contact are obtained by adding the x,y,z outputs of the probe to x,y,z readings of the position of the quill or spindle. Such probes can be used to scan the contour of a workpiece surface, the probe being moved with its stylus in continuous contact with the surface.
It is known to provide such a probe with a linear bearing arrangement for each axis of movement x,y,z. For example, one probe available from the Japanese company Mitutoyo uses a linear air bearing for each axis. This has the advantage of low friction, which is an important factor in a successful measurement probe intended for scanning purposes. However, the use of air bearings not only entails the disadvantage of requiring an air supply. There is also a practical limit to the load capacity and stiffness of an air bearing of a given size, and the biassing springs need to have a relatively low spring rate. Low load capacity limits the size of stylus that can be carried by the probe. Low stiffness limits the static accuracy of the probe, since any deformation will cause an inaccurate reading. Furthermore, the low stiffness and low spring rate affect the probe's dynamic performance. The movable part of this known probe has a relatively low natural frequency of vibration (around 3Hz). This limits the maximum scanning speed at which the probe can scan a workpiece surface with accuracy and without risk of momentary loss of contact.
Another probe has been suggested in EP-A-0128561 (Mecof). Instead of air bearings, this probe uses ball or roller bearings in each of the three axes x,y,z. The x and y axes each have a parallel pair of linear bearings, each comprising a groove in which are located balls or crossed rollers. The z axis has a single cylindrical linear ball bearing. However, in the design of such a probe, one needs to consider the loading capability of the bearings, their stiffness, and their friction. The loading capability affects the ability of the bearing to provide a reaction to the bending moments when the stylus is deflected by contact with the workpiece, and is thus linked to the maximum possible spring rate for the biassing springs. Taking account of these considerations, this proposed probe requires rather large linear bearings in order to achieve the desired dynamic scanning performance, and thus cannot be built into a small housing. Furthermore, the bearings are subject to an unacceptably high degree of friction.
The present invention seeks to overcome some or all of the above problems. The invention has various aspects, as specified in the appended claims.